Method for restricting propellant grains



United States Patent 3,381,065 METHOD FOR RESTRICTING PRGPELLANT GRAINS Reinhart P. Deschner, Seymour, Ind, and Charles J.

Korpics, Oak Lawn, 11]., assignors to Standard Oil Company, Chicago, Ill., a corporation of Indiana N0 Drawing. Filed May 27, 1966, Ser. No. 553,274 7 Claims. (Cl. 264-3) ABSTRACT OF THE DISCLOSURE Improved method for producing polyurethane restricted solid propellant grains wherein the heated, restricted grain is subjected to the additional critical step of repressing, at a pressure in excess of 2,000 p.s.i.

This invention relates to solid propellants having a polyurethane restrictor, and more particularly to an improved method for producing said propellants.

In gas generation for rocket or missile usage, it is desirable that the gas affording composition develop gas at a controlled rate. In order to attain a substantially controlled rate of gas generation, it is necessary to utilize a particular type of configuration for the gas generating composition and to control the burning area of the composition. Unless very special precautions are taken all surfaces of the gas generating composition present in the combustion zone will burn. In solid propellants even the narrowest of fissures will result in two burning surfaces, i.e., one on each side of the fissure. To illustrate, a solid propellant composition in a cylindrical configuration cannot be fitted so tightly against the wall of a combustion chamber that burning of the cylinder surface is prevented, that is, in the absence of some special precaution a cylindrical grain would burn at both ends and the cylindrical surface.

A controlled rate of burning or a controlled change in rate of burning is attained by applying a relatively noncombustible coating to the surface of the propellant body where direct burning is to be prevented. This coating is commonly referred to as a restrictor or combustion restrictor. The requirements for satisfactory restrictors are stringent. In the first instance the restrictor must adhere to the surface of the solid propellant body. In fact, the strength between the propellant and the restrictor should be stronger than the restrictor body itself. Also, the restrictor must be substantially non-porous; the presence of pores or holes in the coat results in combustion of the solid propellant at the point with resultant variation in the gas production rate. Also the restrictor must not develop fissures or cracks under prolonged storage conditions. It is an ordinary military requirement that solid propellants be able to withstand repeated changes of temperature from as much as 65 F. to as much as +160 F. without materially changing the ambient gas production rate.

Polyurethane has been found to be an excellent material for the purpose of restriction and is used extensively; however, polyurethane restricted grains have not met with complete success because of the large number of failures in not being able to withstand such repeated changes of temperature. In the past, rejections of nearly /3 to /2 of such grains were not uncommon because of these failures.

Prior processes involve the application of the restrictor by forcing the fluid polyurethane around the grain under pressure. These processes have been able to form the restrictor in place with good adhesion of restrictor to grain surface, but have inexplicably been inadequate after undergoing the extreme temperature changes because of the appearance of fissures, which as mentioned above, give rise to an uncontrolled gas production.

An improved process for the production of polyurethane restricted propellant has now been discovered. Grains produced by this improved process have met with nearly complete success in their ability to withstand repeated changes of temperature extremes without developing fissures.

Very briefly, the process of the present is described as follows: A propellant composition containing ammonium nitrate as the principal oxidizing agent is molded under pressure. The propellant grain i then placed in a mold adapted to accommodate the application of restrictor; and fluid polyurethane is forced into the mold around said grain and allowed to cure. The restricted grain is then heated and placed in a repress mold and compressed. The restricted propellant grain is heated to a temperature and for a time sufficient to cause softening of the surface of the propellant. The heated grain is then subjected to a pressure of at least 2,000 p.s.i. This additional critical step of repressing has been found to be the difference between failure of a large number of grains and nearly complete acceptance. It has been found that the process of the present invention has the unexpected result of creating a grain capable of withstanding the temperature cycling test much more effectively than polyurethane restricted grains of the past, i.e., fewer failures have been found after temperature cycling the grain produced by the process of this invention.

More specifically, it is advantageous that the restricted grain be heated to a temperature of between about F. to 230 F. for a period of between about 4 to 6 hours. Heating the grain to this temperature will not only soften the surface of the grain, but have the additional advantage of catalytically curing the restrictor. It is preferred to heat the grain to a temperature of between F. to F. The grain may then be subjected to a high pressure in the mold in which it is heated or may then be put into a hot mold and subsequently subjected to pressure. The pressure which may be used to obtain the beneficial results of this process must be at least 2,000 p.s.i. and may be exerted for a time from several seconds until the grain is cooled. The upper limit of the pressure depends, for practical purposes, upon the economics of production. A suitable pressure range would be between 2,000 p.s.i. and 20,000 p.s.i. for a period of between 2 to 4 minutes. It is preferred that the pressure be between 3,000 and 8,000 p.s.i.

If a liner or insulator, such as an asbestos sleeve, is to be used with the propellant, it may be incorporated during the potting procedure. This may be accomplished by placing the asbestos sleeve within the potting mold, and forcing the fluid polyurethane between the liner and the grain and thereby bonding the restrictor to both the grain and the liner. It may also be desirable during the repress procedure to isolate the restricted grain from the repress mold. This may be accomplished by applying a metal coated polyester film (Mylar) to the restricted portion of the grain. Aluminized polyester film is preferable. In addition to protecting the restrictor, such film has the added advantage of facilitating removal from the mold.

Under certain conditions it is desirable to provide an undercoating adhering to the body portion of ammonium nitrate propellant in order to seal the body portion so that the restrictor coating adhered to the undercoating cannot contact ammonium nitrate.

The undercoating is a material capable of adhering firmly to the surface of the ammonium nitrate body portion and also capable of forming an adhesion bond with the restrictor coating material. Also, the undercoating must be sufficiently impervious to seal the surface and bar access of the restrictor coat to said surface. In those cases where the restrictor coating is applied by potting the undercoated grain using a liquid reaction mixture to provide the restrictor coating, the undercoating must be essentially insoluble in the liquid reaction mixture. The undercoating is generally in itself not suitable for use as a restrictor material and therefore is applied in a thickness just enough to provide the necessary sealing effect. The thickness is dependent upon the particular undercoat forming material and may be from V1000 inch to on the order of 3 inch. In those cases where the undercoat material is sufficiently flexible to act as a restrictor a thicker coating may be used.

The solid propellant of the invention comprises a shaped body portion formed of ammonium nitrate as the major component, and an oxidizable binder therefor. This body portion may be any of the configurations commonly used for gas generator purposes or rocket propulsion purposes, for example, a simple cylinder, a tube, a cylinder positioned within a tube, various cruciforms, internal star shaped openings with various types of external surfaces, particularly cylindrical, etc. The polyurethane resistors is positioned immediately contiguous to that part of the surface of the propellant body where direct burning is to be prevented. For example, in a tubular grain the annular ends may be coated with a restrictor in order to force the burning to be on the exterior and internal cylindrical surface only. In other instance only a particular area of a body portion may be coated with a restrictor to provide a very short term control of burning area; for

example, it may be necessary to have all the surface burning and yet immediately after ignition pressure surges must be avoided. This is done by restricting only a small portion of the body control burning for perhaps one-half second and at the end of that time the restrictor coating will be removed by the combustion gases.

Many polyurethane materials are known in the art for use as combustion restrictors. A number of polyurethane restrictors and the preparation thereof are found in US. 3,012,508, see also Dombrow, Polyurethanes (Reinhold 2nd ed. 1965).

By way of example, a particular polyurethane restrictor coating may be produced by a slow setting reaction involving an aromatic diisocyanate, a saturated polyester having terminal hydroxyl groups and a molecular weight between about 600 and 3,000 and a catalyst. The restrictor must be free of holes and thin spots, therefore, precautions should be taken to keep the reaction mixture essentially free of materials that produce foam by reaction with the isocyanate group. For this reason, it is preferred to use a polyester which is essentially free of carboxyl groups. The isocyanate groups are present in an amount of at least theoretical equivalent for reaction with the hydroxyl groups of the polyester; an excess is preferable and as much as 1.5 times the theoretical requirement may be used.

The catalyst used in the preparation of the polyurethane is a slow acting catalyst. A slow acting catalyst is necessary because the manner of producing a restrictor coating requires flow of the reaction mixture through narrow void spaces between the surface of the propellant body and the shell positioned at the portion of the body which is to be restricted. The thickness of the restrictor coating will be determined by the particular requirements. In general nonporous restrictors are obtained in coats as thin as ,4 inch. It is usual to use a thicker restrictor coat and in general the coat will be between about and W inch thick, circumstances may require coating inch or more. It is to be understood that the restrictor coat should be no thicker than the requirements of the particular application since excess thickness of material results in uneconomic costs and added weight. In general, it is preferred that the set time of the reaction mixture in a beaker containing about 100 grams of polyester of molecular weight about 1,700 with grams of toluene diisocyanate (80:20 commercial mixture) and 1 gram of catalyst intermingled at -80 F. be at least about 30 minutes; the timing being determined from the moment of adding the catalyst to the mixture of isocyanate compound and polyester and the moment when the material in the beaker is too thick to flow appreciably when the beaker is turned on its side. It is to be understood that the set time of the reaction mixture will be determined by the particular application; a long relatively thin void space may require a far slower setting mixture than a short wide void space. In general, for ease in operation, the catalyst type and usage will be determined to give a setting time only a few moments longer than the time needed for the reaction mixture to flow into the space farthest from the point of introduction of the liquid reaction mixture.

Suitable catalyst for the practice of the invention are selected from the class consisting of N-cocomorpholincs, pyridines, quinolines, isoquinolines, and ethoxylated amines. In addition to pyridine, quinolines and isoquinoline per se various substituted members of these compounds are useful as catalysts in the process. The usage of catalyst is determined by the particular reactants and by the setting time requirements. The amount of catalyst, based on parts by Weight of polyester, may vary from as little as 0.1 or less to as much as 10 parts by Weight. It is usual to use between about .25 part and 3 parts by weight.

The propellant body of the invention contains ammonium nitrate as the major component. Ammonium nitrate compositions consist essentially of ammonium nitrate particles in an oxidizable organic material (binder) and various other additives such as catalyst for the promotion of combustion, carbon, chemical stabilizer to reduce decomposition, etc. To improve the burning quality, and to utilize the excess free oxygen made available by the decomposition of the ammonium nitrate, oxidizable organic material, which also may function as binder material for the shaping of the ammonium nitrate into grains, are admixed With ammonium nitrate. These oxidizable organic materials may be any thermoplastic known to the art for use in propellant compositions.

The multi-component binder or matrix former commonly consists of a polymeric base material and a plasticizer therefor. Particularly suitable polymeric base materials are cellulose esters of alkanoic acids containing from 2-4 carbon atoms such as cellulose acetate, cellulose butyrate and cellulose propionate.

The polyvinyl resins such as polyvinyl chloride and polyvinyl acetate are also good bases; styrene acrylonitrile is an example of a copolymer which forms a good base material; polyacrylonitrile is another suitable base material; as are polyamide resins (such as nylons).

The plasticizer component of the binder also, preferably, contains combined oxygen. The oxygen may be present in the plasticizer as an ether linkage and/or hydroxyl and/ or carbonyl; also the oxygen may be present as an inorganic substituent, particularly a nitro group.

In general, any plasticizer which is adapted to plasticize the particular polymer may be used. A single plasticizing compound may be used; more usually 2 or more compounds are used in conjunction; for example, acetyl triethyl citrate and triethyl citrate, etc.

The particular requirements with respect to use will determine not only the polymer but also the particular plasticizer or combination of plasticizers which are used. The precise amount of binder is dependent upon the type of material forming the binder as well as the requirements for the particular grain.

Example Several temperature shock-cycling tests (hereinafter described) were performed on an ammonium nitrate composition of the following formulation: 62.3% ammonium nitrate, 11.4% cellulose acetate, 21.1% plasticizers, 3.0% carbon black, 1.0% sodium barbiturate, 1.2% stabilizers.

After mixing, the above formulation was heated to 100- C. and molded under pressure. An undercoating solution of 1 part by weight dried cellulose acetate and 9 parts by weight acetone was applied to the surface and allowed to dry. A second undercoating solution was then applied comprising parts of toluene diisocyanate and 0.5 part of N-cocomorpholine to 100 parts of a solution prepared from 1 part cellulose acetate and 9 parts acetone. The propellant was then heated and inserted into a potting mold. A restrictor coating was then prepared from the following formulation: 100 parts Pittsburgh Plate Glass Selectron Resin #6203 (adipate based polyester hydroxyl-terminated prepolymer, average molecular weight of 1700) was stirred for 6 minutes with part N-cocomorpholine and afterwards allowed to stand for 5 minutes. Toluene diisocyanate was stirred in for 5 minutes in an amount equal to 1.15 times theoretical (based on hydroxyl number). An asbestos sleeve Was placed in the potting mold. The restrictor material was forced upward between the propellant and the asbestos sleeve inside the potting mold until it completely covered the top of the grain. The resin was cured for 16 hours after which the grain was removed from the mold and an aluminized Mylar film was applied to the restricted portion of the grain. It was placed in an oven for 5 hours and maintain at a temperature of 150:10 F. The grain was then placed in a repress mold and pressed at 4,000 p.s.i. for 3 minutes, while the grain was still hot, i.e., above about 130 F.

Temperature shock cycling test The grains selected for the hot firings were placed in a temperature conditioning chamber having an internal temperature of 65i5 F., and the grains selected for the cold firings were placed in a temperature conditioning chamber having an internal temperature of +165 F. After remaining in the respectvie chambers for 511 hours the grains were removed from its conditioning chamber and immediately placed in the other chamber for a like period of time. After remaining for the specified period of time in a temperature chamber, the grains were again ready for temperature chamber interchange. A minimum of 4 hours at one temperature extreme, followed by a minimum of 4 hours at the other temperature extreme constitutes one complete temperature-shock cycle. The grains of the above example were subjected to 10 complete temperature shock cycles.

Twenty lots prepared by the process of this invention had no failures due to propellant cracking during cycling; whereas one-third of the lots of the same formulation prepared by the same process but without the critical steps of this invention exhibited failure due to propellant cracking.

This invention is even more effectively demonstrated when a 20 cycle test is used. One hundred percent of all grains produced by this process and tested at 20 cycles have passed the test, whereas grains produced by prior processes met with a failure rate of more than 30%.

We claim:

1. A process for the preparation of a propellant grain comprising a propellant composition containing ammonium nitrate as the primary oxidant, which process comprises molding said composition under pressure to form a grain, applying a polyurethane combustion restrictor to the surface of said grain to be restricted, at least partially curing said restrictor, heating said restricted composition to a temperature sufiicient to cause softening of the surface of said grain, and subjecting said heated restricted grain to a repressing step at a pressure of at least 2,000 p.s.i

2. The process of claim 1 wherein said pressure applied to said restricted grain is in the range of 2,000 to 20,000 p.s.i.

3. The process of claim 1 wherein said pressure applied to said restricted grain is in the range of 3,000 to 8,000 p.s.i.

4. The process of claim 1 wherein said restricted composition is heated to a temperature in the range of to 230 F.

5. The process of claim 1 wherein said restricted composition is heated to a temperature in the range of to F.

6. In a process for the production of a polyurethane restricted propellant grain comprising a propellant composition containing ammonium nitrate as the primary oxidant, which process comprises molding said composition under pressure to form a grain, applying a polyurethane combustion restrictor to the surface of said grain to be restricted, at least partially curing said restrictor, the improvement which comprises heating said grain to a temperature sufiicient to cause softening of the surface of said grain, and subjecting said heated restricted grain to a repressing step at a pressure of at least 2,000 p.s.i.

7. The process of claim 6 wherein said restricted grain has applied to the restricted portion of said grain, an aluminized polyester film prior to being subjected to said pressure.

References Cited UNITED STATES PATENTS 2,977,884 4/1961 Mahon et al 149-7 X 2,987,388 6/1961 Stanley 149-7 X 3,010,354 11/1961 Adelman 861 3,046,829 7/1962 Roemer 264-3 X 3,107,573 10/1963 Butcher 86-1 3,188,962 6/1965 Mosher 264-3 X 3,215,028 11/1965 Pitchford et al 86-1 3,260,631 7/1966 Witz et al 149-7 3,263,613 8/1966 Rice et a1. 264-3 X 3,301,924 1/1967 Bryant et al. 2643 CARL D. QUARFORTH, Primary Examiner. M. J. SCOLNICK, Assistant Examiner. 

